Method for producing a hard coating with high corrosion resistance on articles made anodizable metals or alloys

ABSTRACT

A method for coating, a composition suitable for coating and a coating generated with the method of coating on anodizable metallic surfaces, especially on magnesium rich and aluminum rich surfaces is disclosed. The composition is an aqueous solution including alkali metal or ammonium cations, phosphorus containing anions and silicon containing anions as well as optionally a peroxide or a compound of Al, Ti, Zr or any mixture of them. Preferably, the anodizing is carried out with a micro-arc oxidation process.

FIELD OF THE INVENTION

The present invention is directed to the field of metal surfacepreparation utilizing anodizing processes with aqueous compositionssuitable for the anodizing of anodizable metallic materials. Inpreferred embodiments the invention relates to a method and acomposition of anodizing surfaces of anodizable metallic materials by amicro-arc oxidation process especially of surfaces of magnesium,magnesium alloys, aluminum, aluminum alloys or these mixtures or ofsurfaces or surfaces' mixtures containing such metallic materials.

BACKGROUND OF THE INVENTION

The light weight and strength of magnesium and magnesium alloys makesproducts fashioned therefrom highly desirable for use in manufacturingcritical components to be used, for example, for aircraft, forterrestrial vehicles or for electronic devices. But the most significantdisadvantage of magnesium and magnesium alloys that they easily corrode.The exposure of such metallic materials' surfaces to a chemicallyhazardous environment causes that their surfaces corrode rather quicklyand strongly. Corrosion is both unesthetic and reduces strength.

There are many methods known for improving the corrosion resistance of aworkpiece of magnesium and magnesium alloy by modifying the surface ofthe workpiece. It is generally accepted that the best corrosionresistance for magnesium and magnesium alloy surfaces is achieved byanodizing. In the anodizing process, a metallic workpiece is used as ananode or with an alternating current as an anode and as a cathodealternating according to the frequency of the alternating current of anelectrical circuit, the circuit including an electrolyte bath in whichthe workpiece is at least partially immersed. Depending on theproperties of the current, the bath temperature and the composition ofthe solution of the electrolyte bath, the surface of the workpieces maybe modified in various ways. The metallic workpiece (substrate, article)may be a coil, a sheet, a wire, a workpiece made from a coilrespectively from a sheet or a more or less massive part with a simpleor complex shape.

Various solutions and additives are found for example in: U.S. Pat. No.5,792,335 discloses ammonia and phosphate containing electrolytesolutions with an optional content of ammonium salt and peroxide; U.S.Pat. No. 6,280,598 teaches electrolyte solutions that may containdifferent amines or ammonia and phosphate or fluoride and subsequently asealing agent may also be applied; WO 03/002773 describes electrolytesolutions containing phosphate, hydroxylamine and alkali metalhydroxide. The anodizing methods disclosed in these publications allow alayer comprising magnesium hydroxide and magnesium phosphate. Theseanodizing processes offer high corrosion resistance.

Although anodizing is effective in increasing the corrosion resistance,the hardness and the scratch resistance of the surfaces are ofteninsufficient especially for anodizing coatings generated on the surfaceof magnesium rich material, primarily because a high concentration ofmagnesium hydroxide in the generated anodizing coatings. In conventionalanodizing processes even on the surfaces of materials rich in aluminum,beryllium, iron or titanium, the generated anodizing coatings aretypically rich in at least one hydroxide and therefore not as hard asexpected. On the other hand, the processes of anodizing based on acidicelectrolyte solutions do not offer a sufficiently high corrosionresistance.

One of the ways to solve this problem is to apply a coating rich inceramic oxides especially by micro-arc electrolytic oxidation process.

The investigation of micro-arc electrolytic oxidation for light metalshas continued for more than fifty years. The micro-arc oxidation methodhas several names: Micro-arc oxidation, micro-plasmic oxidation,plasma-liquid coating, etc. Methods and compositions to apply a ceramicoxide coating by anodizing on aluminum have been disclosed in severalpublications: SU 1200591 teaches to build an oxide coating with highhardness and wear resistance in alkaline solutions of potassiumhydroxide, “liquid glass” (=water glass) and sodium aluminate. Analternating current with a frequency of about 50 Hz and with a currentdensity in the range from 0.5 to 24 A/dm² (current density of thecathodic phase) and in the range from 0.6 to 25 A/dm² (current densityof the anodic phase) is supplied to the metallic material. DE 42 09 733teaches an anode-cathode oxidation in an alkali metal silicate or in analkali metal aluminate electrolyte solution. Pulses with a frequency inthe range from 10 to 150 Hz are used. The method offers solid oxidecoatings with a thickness in the range from 50 to 250 microns andrequires a very high energy consumption and a complex equipment. U.S.Pat. No. 5,616,229 discloses a method of obtaining a ceramic oxidecoating on aluminum. The method uses again potassium hydroxide andsilicate in the electrolyte solution.

A general drawback of alkali metal hydroxide and silicate containingelectrolyte solutions is the low stability of the said electrolytesolutions. By applying the typical electricity for such a process, theelectrolyte solution changes within a short time—especially after theuse from about 30 to about 90 A·h/L to a kind of gel because of the highpolymerization of the solution and should therefore be completelyreplaced.

U.S. Pat. No. 4,659,440 teaches a method of coating aluminum articles inelectrolyte solutions comprising an alkali metal silicate, a peroxide,an organic acid and a fluoride. A vanadium compound may also be includedfor decorative purposes. U.S. Pat. No. 5,275,713 discloses a method ofcoating aluminum surfaces with an electrolyte solution containing alkalimetal silicate, an organic acid, potassium hydroxide, a peroxide, afluoride and molybdenum oxide. The voltage is first raised to 240 to 260V and then increase the voltage to a range from 380 to 420 V. U.S. Pat.No. 5,385,662 teaches a method of producing oxide ceramic layers onbarrier layer-forming metals which include aluminum or magnesium richmetallic surfaces. The electrolyte solutions contain ions of phosphate,borate and fluoride.

A main drawback of the described electrolyte solutions described inthese publications is the content of hazardous components like fluoridesand heavy metals.

RU 2070622 and U.S. Pat. No. 6,365,028 disclose methods for producingceramic oxide coatings on aluminum in electrolyte solutions comprisingan alkali metal hydroxide, an alkali metal silicate and an alkali metalpyrophosphate. An alternating current with a frequency in the range from50 to 60 Hz is supplied to the metal. The addition of pyrophosphate ionsto the classic combination of alkali metal hydroxide and silicateimproves the stability of the electrolyte solution. In order toaccelerate the oxide layer formation, the inventor used peroxideadditives in the second patent publication mentioned here. A drawback ofthe disclosed method is the high content of the alkali metal hydroxidethat is undesirable for magnesium rich surfaces because of high contentsof magnesium hydroxide in the generated coatings.

A high content of an alkali metal hydroxide in the electrolyte solutionaccelerates the formation of magnesium hydroxide and magnesium oxide onthe metallic surfaces and assists in producing coatings with a lowhardness and with a low stability against acids. Additionally, asignificant content of at least one metal hydroxide seems to reduce thestability of the silicate containing electrolyte solutions severely.U.S. Pat. No. 4,978,432 teaches to produce protective coatings that areresistant to corrosion and wear on magnesium and magnesium alloys. Theelectrolyte solutions comprise ions of borate or sulfonate, phosphateand fluoride or chloride. The obtained coatings include magnesiumphosphate and magnesium fluoride and optionally magnesium aluminate thatoffer good corrosion and wear resistance. However, the electrolytesolutions are not sufficiently environmentally friendly.

A method that is similar to the proposed invention is disclosed in SU1713990. It teaches a method of micro-arc anodizing for metals inalkaline electrolyte solutions. The anodizing is performed by anasymmetric AC current so that the hardness is increased because of agood sintering. The current density is decreased by steps in the rangefrom 20 to 60%. The disclosed compositions which include sodiumhexametaphosphate (Na₆P₆O₁₈) do not include a second phosphoruscontaining compound and no addition of any alkali metal hydroxide. Amain drawback of the disclosed method is the complex electrical controland the low rate of the coating formation. The method has not beenadapted and not optimized for magnesium rich surfaces.

WO 03/002773 discloses a method of anodizing magnesium surfaces inalkaline phosphate solutions. The method allows to build quicklyanodizing layers that contain a magnesium phosphate. The generatedlayers offer excellent corrosion resistance and good adhesion. Thecoating method was approved for application in aircraft industries.However, the coatings have a low hardness because of a high content ofmagnesium oxide and magnesium hydroxide.

It would be highly advantageous to have a method for treating thesurfaces of anodizable metallic materials and especially magnesium ormagnesium alloy surfaces so as to generate coatings of a high hardnessand of a high corrosion resistance. Further on, it is preferable thatsuch a treatment is environment friendly and does not include aconsiderable content of fluorides, heavy metals and other hazardouscomponents. It would be favorable if this process would be not toocomplex and not too expensive.

SUMMARY OF THE INVENTION

The present invention relates to a composition of an aqueous electrolytesolution useful for the oxidation of a surface of at least oneanodizable metallic material with a pH greater than 6 comprising:

-   -   i. at least two different phosphorus containing compounds having        different anions which are at least partially soluble in the        aqueous solution used, at least a first being called        component a) and at least a second being called component b);    -   ii. at least one silicon containing compound which is at least        partially soluble in the aqueous solution used; and    -   iii. an amount of at least one type of cations selected from        alkali metal cations and ammonium cations;    -   iv. whereby the electrolyte solution shows a total concentration        of at least one hydroxide of Na, K, Li, NH₄ or any mixture of        these intentionally added to the electrolyte solution below 0.8        g/L or whereby the electrolyte solution is free of any hydroxide        of Na, K, Li, NH₄ or any mixture of these added intentionally.

The present invention relates further on to a composition of an aqueouselectrolyte solution useful for the oxidation of a surface of at leastone anodizable metallic material with a pH greater than 6 comprising:

-   -   i. at least two different phosphorus containing compounds        showing different anions which are at least partially soluble in        the aqueous solution used, at least two of them being called        component a) and component b), wherein there is contained a        moiety of at least one phosphorus containing compound showing        oxyanions;    -   ii. an amount of at least one compound selected from organic        silicates, inorganic silicates, silicon containing oxides,        silanes, silanols, siloxanes and polysiloxanes, their        derivatives or any mixture of them that are sufficiently stable        in the electrolyte solution, essentially non-toxic and        water-soluble or at least partially water-soluble;    -   iii. a moiety (compound) of at least one of the cations of Na,        K, Li, NH₄ or any mixture of these;    -   iv. whereby the electrolyte solution shows a total concentration        of at least one hydroxide of Na, K, Li, NH₄ or any mixture of        these intentionally added to the electrolyte solution below 0.8        g/L or whereby the electrolyte solution is free of any hydroxide        of Na, K, Li, NH₄ or any mixture of these added intentionally.

The present invention relates additionally to a method of treating ametallic workpiece comprising:

-   -   a) providing a metallic surface chosen from metallic surfaces of        at least one metallic material that may be anodized;    -   b) immersing said surface in an electrolyte solution whereby the        solution may really be a solution, a sol, a gel, a suspension or        any mixture of them;    -   c) providing at least one electrode in said electrolyte        solution; and    -   d) passing a current between said surface and said electrode        through said electrolyte solution wherein said electrolyte        solution is an aqueous solution with a pH greater than 6 that        has a composition according to the invention. As noted above,        the electrolyte solution may take different forms, such as a        solution, a sol, a gel, a suspension or any mixture of them; and        the term electrolyte bath may be used to refer to any such form.

The present invention relates even to a protective coating produced by amethod according to the invention.

The present invention relates finally to a method of use of a metallicworkpiece coated with a protective coating which is produced by a methodaccording to the invention for aircrafts, for terrestrial vehicles orfor electronic devices.

DETAILED DESCRIPTION OF THE INVENTION

The present invention concerns a method of treating a metallic workpiecein an electrolyte solution by anodizing, a composition useful for suchanodizing and a coating generated therewith whereby the anodizing isfavorably carried out with a micro-arc oxidation process, especially onmagnesium rich or on aluminum rich surfaces. The composition is anaqueous solution including i) at least two phosphorus compounds like acombination of an orthophosphate and a pyrophosphate, ii) at least onesilicon containing compound like an alkali metal silicate, iii) acontent of at least one alkali metal compound or ammonium compound orboth and optionally a) a not too high content of at least one hydroxide,b) a peroxide or c) at least one compound comprising atoms of Al, Ti, Zror any mixture of these chemical elements resp. a combination of suchsilicon respectively aluminum, titanium, zirconium or any combination ofthem containing compounds or any combination of compounds selected fromthe group consisting of a), b) and c).

Compositions of the Electrolyte Solution of the Present Invention

The compounds mentioned herein may be present in the electrolytesolution in the form of compounds, of their ions or of both of them.

The composition of the electrolyte solution contains preferably a moietyof at least one type of anions selected from phosphorus containingoxyanions.

The composition of the electrolyte solution contains preferably a moietyof at least one primary phosphate, of at least one secondary phosphate,of at least one orthophosphate, of at least one condensed phosphate likeof at least one metaphosphate or of at least one polyphosphate or ofboth, of at least one pyrophosphate, of at least one phosphonate, of atleast one phosphonite, of at least one phosphite, of at least onederivative of them or of any mixture of them.

The composition of the electrolyte solution contains preferably:

As component a) a moiety (compound) of at least one primary, secondaryor tertiary phosphate or of at least one derivative of them or of anymixture of them and as component b) a moiety (compound) of at least onepyrophosphate or of at least one derivative of it or of any mixture ofthem.

The composition of the electrolyte solution contains preferably at leastone of said phosphorus containing compounds chosen from the groupconsisting of K₃PO₄, Na₃PO₄, (NH₄)₃PO₄, K₂HPO₄, Na₂HPO₄, (NH₄)₂HPO₄,KH₂PO₄, NaH₂PO₄, NH₄H₂PO₄, K₄P₂₀₇, Na₄P₂O₇ and (NH₄)₄P₂O₇. It is clearto one skilled in the art that alternatively or additionally to theseother phosphates that are sufficiently soluble in the electrolytesolution may be incorporated in the electrolyte solution.

The electrolyte solution of the present invention contains preferably atleast one alkali metal pyrophosphate or ammonium pyrophosphate or both,preferably added as at least one water-soluble phosphate salt, morepreferred selected from potassium pyrophosphate (K₄P₂O₇), sodiumpyrophosphate (Na₄P₂O₇) and any mixture of these. The totalconcentration of said pyrophosphate(s) is preferably in the range from0.001 to 2 M/L or is preferably in the range from 0.1 to 240 g/L, e.g.preferably 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45, 48,51, 54, 57 or 60 g/L.

An electrolyte solution with a too high concentration of the phosphoruscontaining compounds may provide thick, fragile coatings. An electrolytesolution with too low of a concentration of the phosphorus containingcompounds may form inhomogeneous unesthetic layers, especially oncomplex forms of workpieces like such with deepenings recesses, orconcavities. An electrolyte solution with a too high concentration ofhydrophosphate or of pyrophosphate or of both may provide thick, fragilecoatings. An electrolyte solution with a too low concentration ofhydrophosphate or of pyrophosphate or of both may be of a relatively lowpH and may form inhomogeneous unesthetic layers and in some cases theelectrolyte solution may earlier alter to a gel like composition.Although not intending to be bound to any of the theories of anodizingtechnologies, it is believed that the presence of the pyrophosphate ionsin the electrolyte solution of the present invention contributes to thestability of the electrolyte solution, that means that the life time ofthe electrolyte solution is not too much altered to a thickened gel likecomposition.

The crystal water content of these compounds may be e.g. zero or asusually known for the respective compound or intermediate between suchdata. In the calculations, the water content of such compounds has to beconsidered, too, even if it is not mentioned in the formulas of thistext.

The composition of the electrolyte solution contains preferably the atleast two phosphorus containing compounds in a total concentration inthe range from 0.2 to 250 g/L, more preferred in the range from 0.5 to180 g/L, most preferred in the range from 1 to 120 g/L, often in therange from 2 to 80 g/L, whereby the concentration is calculated underconsideration of a crystal water content if present. This totalconcentration may especially be e.g. 3, 6, 9, 12, 15, 18, 21, 24, 27,30, 33, 36, 39, 42, 45, 48, 51, 54, 57, 60, 65, 70, 75, 80, 85 or 90g/L.

The composition of the electrolyte solution contains the at least twophosphorus containing compounds preferably in a total amount in therange from 0.001 to 2 M/L, more preferred in the range from 0.02 to 1.2M/L, most preferred in the range from 0.05 to 0.8 M/L, often in therange from 0.01 to 0.5 M/L, whereby the concentration is calculatedunder consideration of a crystal water content if present.

The composition may preferably contain said component a) in aconcentration in said electrolyte solution in the range from 0.1 to 220g/L and may preferably contain said component b) in said electrolytesolution in the range from 0.1 to 220 g/L, more preferred the componenta) in the range from 0.2 to 160 g/L, most preferred in the range from0.3 to 100 g/L, often in the range from 0.5 to 75 g/L, and morepreferred the component b) in the range from 0.2 to 160 g/L, mostpreferred in the range from 0.3 to 100 g/L, often in the range from 0.5to 75 g/L, whereby the concentration is calculated under considerationof a crystal water content if present. The concentration of saidcomponent a) or of said component b) may especially be e.g. 3, 6, 9, 12,15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45, 48, 51, 54, 57 or 60 g/L.

The composition may preferably contain said component a) in aconcentration in said electrolyte solution in the range from 0.002 to1.8 M/L and may preferably contain said component b) in said electrolytesolution in the range from 0.002 to 1.8 M/L. More preferred, thecomponent a) is contained in the range from 0.0012 to 1.4 M/L, mostpreferred in the range from 0.003 to 1 M/L, often in the range from0.005 to 0.5 M/L. More preferred, the component b) is contained in therange from 0.0012 to 1.4 M/L, most preferred in the range from 0.003 to1 M/L, often in the range from 0.005 to 0.5 M/L. The concentration iscalculated with a crystal water content if present.

A low phosphate concentrated electrolyte solution may provide a hardercoating but sometimes with a less high corrosion resistance. A highphosphate concentrated electrolyte solution may provide a thick, fragilecoating with a lower hardness, but often with a high corrosionresistance.

The composition of the electrolyte solution contains preferably a moietyof at least one sodium containing silicate, at least one potassiumcontaining silicate, at least one ammonium containing silicate, at leastone of their derivatives or any mixture of them. The composition of theelectrolyte solution may contain any amount of at least one alkali metalsilicate, preferably of a sodium or a potassium silicate, morepreferably added as “liquid glass”.

The composition of the electrolyte solution contains preferably a moietyof at least one alkali metal silicate or of any monomer, of any polymeror of even both of any silicon containing compound like any silane, anysilanol, any siloxane or any polysiloxane or at least one of theirderivatives or any mixture of them. Favorably, this composition containsat least one compound chosen from sodium containing silicate, sodiumcontaining silicon oxide, potassium containing silicon oxide andpotassium containing silicate. Alternatively or additionally to at leastone other silicon containing compound, it is preferred to add anysilicon containing sol or gel e.g. on the base of at least one alkalimetal silicate like water glass.

The composition may preferably contain a total concentration of the atleast one silicon containing compound in said electrolyte solution inthe range from 0.5 g/L to 70 g/L, more preferred in the range from 1 to50 g/L, most preferred in the range from 1.5 to 30 g/L, often in therange from 2 to 15 g/L, whereby the concentration is calculated underconsideration of a crystal water content if present. This totalconcentration may especially be e.g. 3, 6, 9, 12, 15, 18, 21, 24, 27,30, 33, 36, 39, 42, 45, 48, 51, 54, 57 or 60 g/L.

Too high a concentration of the at least one silicon containing compoundin the electrolyte solution may provide fragile coatings. Furthermore, ahigh concentration of the at least one silicon containing compound inthe electrolyte solution may accelerate its polymerization and maytruncate the life time of the electrolyte solution. To low of aconcentration of the at least one silicon containing compound in theelectrolyte solution may provide less hard coatings. In certaininstances, especially on aluminum poor or aluminum free metallicsurfaces, the hardness of the generated coating is at least by a greaterextent determined by the content of silicon oxide(s) if there should bea low content of aluminum oxide(s).

The composition may preferably contain a total concentration of the atleast one silicon containing compound in said electrolyte solution inthe range from 0.001 to 2 M/L, more preferred in the range from 0.003 to1.4 M/L, most preferred in the range from 0.007 to 0.8 M/L, often in therange from 0.01 to 0.5 M/L, whereby the concentration is calculatedunder consideration of a crystal water content if present.

The composition may preferably contain a total concentration of at leastone hydroxide of Na, K, Li or NH₄ or of any mixture of them of no morethan 0.8 g/L in the electrolyte solution, more preferred no more than0.6, 05. or 0.4 g/L or even no more than 0.3, 0.2 or 0.1 g/L oroptionally none. This concentration may show, but must not show onlyintentionally added moieties, but may even enclose moieties that aredragged in in the process succession e.g. from an earlier bath or thatare impurities of other components or both. The hydroxide may be—atleast partially—contained as anions; then it may be preferred that thecontent of OH-anions shows a concentration that corresponds ascalculated in molar weights to the concentration of the hydroxidesmentioned in this paragraph. The concentration of OH-anions in theelectrolyte solution may be significantly smaller than the concentrationof cations of Na, K, Li or NH₄ or of any mixture of them, e.g. less than80% or less than 60% or less than 40% or even less than 20%.

The composition of the electrolyte solution may preferably show a totalconcentration of cations and compounds of Na, K, Li or NH₄ calculated asNa, K, Li or NH₄ of no more than 0.3 M/L, more preferred no more than0.225 or 0.15 M/L or even no more than 0.075 M/L or optionally none.

Amongst the cations of Na, K, Li, NH₄ or any mixture of them, thecontent of ammonium cations is generally less favorable because it seemsthat it does not take a significant part in the formation of thecoating. Because of environmental reasons, it may be preferred to use acontent or a higher content of potassium cations instead of e.g. sodiumcations.

The composition of the electrolyte solution may contain alkaline earthmetal cations preferably in a concentration of no more than 3 g/L, morepreferred of no more than 2.5 or 2 g/L or even of no more than 1.5, 1 or0.5 g/L or optionally none.

There may be a moiety of alkaline earth metal compounds respectively ofalkaline earth metal cations in the electrolyte solution. It ispreferred that this moiety of alkaline earth metal cations present inthe electrolyte solution is kept in a range from 0.001 to 3 g/L, morepreferred in a range of up to 2 g/L or up to 1.5 g/L, most preferred ina range of up to 1 g/L or up to 0.5 g/L. These alkaline earth metalcations in the electrolyte solution are preferably such cations likecalcium, magnesium or any of their mixtures. The content of alkalineearth metal cations may be integrated into the coatings to a highpercentage or even totally. Of course, a similar content may occur frommagnesium rich surfaces by chemical, electrochemical or thermal reactionor any mixture of these. Nevertheless, it may in some cases be preferredthat the addition of such cations is kept quite low or even zero.

The composition of the electrolyte solution may contain transition metalcations preferably in a concentration of no more than 3 g/L, morepreferred of no more than 2.5 or 2 g/L or even of no more than 1.5, 1 or0.5 g/L or optionally none.

There may be a moiety of transition metal compounds including lanthanidecompounds respectively of transition metal cations in the electrolytesolution. It is preferred that this moiety of transition metal cationspresent in the electrolyte solution is kept in a range from 0.001 to 3g/L, more preferred in a range of up to 2 g/L or up to 1.5 g/L, mostpreferred in a range of up to 1 g/L or up to 0.5 g/L. These transitionmetal cations in the electrolyte solution are preferably such cationslike cerium, iron, manganese, niobium, yttrium, zinc or any of theirmixtures. The content of alkaline earth metal cations may be integratedinto the coating to a high percentage or even totally. Of course, asimilar content may occur from iron or titanium rich surfaces bychemical, electrochemical or thermal reaction or any mixture of these.Nevertheless, it may in some cases be preferred that the addition ofsuch cations is kept quite low or even zero.

The composition of the electrolyte solution may contain anions otherthan oxides, phosphorus containing oxyanions and silicates preferably ina concentration of no more than 3 g/L, more preferred of no more than2.5 or 2 g/L or even of no more than 1.5, 1 or 0.5 g/L.

There may be a moiety of compounds showing anions other than phosphoruscontaining oxyanions or silicon containing oxyanions in the electrolytesolution like an aluminate, a carbonate, a carboxylate, a titanate, azirconate or any mixture of these. It is preferred that this moiety ofanions added to or present in the electrolyte solution is kept in arange from 0.001 to 3 g/L, more preferred in a range of up to 2 g/L orup to 1.5 g/L, most preferred in a range of up to 1 g/L or up to 0.5g/L. The content of these anions may be integrated into the coating to ahigh percentage or even totally, but the decomposition of organic anionsand, e.g., of carbonates will then lead in such cases to a loweredamount in the coating. Nevertheless, it may in some cases be preferredthat the addition of such anions is kept quite low or even zero.

The composition of the electrolyte solution may contain anions ofmineral acids or organic acids other than oxides, phosphorus containingoxyanions and silicates preferably in a concentration of no more than0.2 M/L, more preferred of no more than 0.12 M/L or even of no more than0.6 M/L.

The composition of the electrolyte solution may additionally contain atleast one peroxide. The peroxide may be used as source for oxygen forthe oxidation especially of the base metal going to be anodized. Thesaid peroxide may preferably be hydrogen peroxide, sodium peroxide,potassium peroxide or any mixture of them. Alternatively, other sourcesof oxygen may be used instead of peroxide or additionally to it, butperoxide is favored because it is environmentally very friendly.

The composition of the electrolyte solution may preferably contain theat least one peroxide additionally contained in the electrolyte solutionin a concentration preferably in the range from 0.01 g/L to 20g/L-calculated as 100% of H₂O₂, more preferred in the range from 0.03 to14 g/L, most preferred in the range from 0.06 to 8 g/L, often in therange from 0.1 to 2 g/L. The electrolyte solution of the presentinvention may optionally contain a peroxide like hydrogen peroxide. Theconcentration of said hydrogen peroxide is preferably in the range from0.01 to 50 g/L calculated in the form of 20 to 30% H₂O₂ or preferably inthe range from 0.03 to 20 g/L calculated in the form of 100% H₂O₂.

If it is intended to add any peroxide, it is preferred that there is acertain content of it in the electrolyte solution as the peroxide may beconsumed by chemical reactions in a certain amount during the anodizing.Nevertheless, it is not necessary to add very high amounts ofperoxide(s).

Oxygen provided by the dissociation of the peroxide may accelerate theplasma-chemical reactions and may often improve the properties of thegenerated coating which may gain the properties of a ceramic coating,especially if there is a sintering during the anodizing. However, a toohigh concentration of peroxide may sometimes decrease the stability ofthe electrolyte solution significantly because of the gelling effect ofthe electrolyte solution. Generally, the addition of peroxide or anyother oxygen delivering compound is optional but it is recommended whencompounds of Al, Ti or Zr are added because of the high sinteringtemperature of the oxides of said chemical elements. Therefore, theperoxide additive is recommended in order to reach a high sinteringrate. Additionally, the use of the sol-gel structures of said compoundsmay help to decrease the sintering temperature necessary or favorable togenerate an excellent ceramic coating. If there is no or an insufficientcontent of peroxide, these favorable effects are not to be observed orare lowered.

The composition of the electrolyte solution may contain at least onecompound containing atoms of Al, Ti, Zr or any mixture of these atoms orany mixture of these compounds additionally contained in the electrolytesolution which is water-soluble or which is water-insoluble. Suchwater-insoluble compound(s) may be contained in the electrolyte solutionin the form of particles showing a particle size distribution for allthese particles preferably essentially in the range from 0.01 to 20microns, more preferred essentially in the range from 0.05 to 18microns, most preferred essentially in the range from 0.1 to 15 microns,often essentially in the range from 0.5 to 12 microns. The wording“essentially” shall mean that there must not be 100% of the particlesize distribution within the ranges mentioned, but a main proportion ofit, e.g. at least more than 50%, e.g., 65% or more of the particle sizedistribution calculated by particle numbers.

The composition may preferably contain the at least one compoundcontaining atoms of Al, Ti, Zr or of any mixture of these atoms or ofany mixture of these compounds additionally contained in the electrolytesolution in a total concentration in the range from 0.01 g/L to 50 g/L,more preferred in the range from 0.03 to 30 g/L, most preferred in therange from 0.06 to 10 g/L, often in the range from 0.1 to 1 g/L, e.g. 3,6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45 or 48 g/L.

The composition may preferably contain the at least one compoundcontaining atoms of Al, Ti. Zr or of any mixture of these atoms or ofany mixture of these compounds additionally contained in the electrolytesolution in the range from 0.0001 to 1 M/L, more preferred in the rangefrom 0.0005 to 0.5 M/L, most preferred in the range from 0.001 to 0.2M/L, often in the range from 0.005 to 0.05 M/L.

The composition wherein said at least one compound comprising atoms ofaluminum is preferably at least one aluminate like sodium aluminate orpotassium aluminate or both of them, whereby the aluminate(s) may becontained in the electrolyte solution in a concentration of all of theseat least one of these aluminates preferably in the range from 0.1 g/L to50 g/L, more preferred in the range from 1 to 30 g/L, e.g. 3, 6, 9, 12,15, 18, 21, 24 or 27 g/L.

The composition of the electrolyte solution may contain as solvent a)water or b) water and at least one alcohol, preferably c) only water andethanol or d) water and a glycol such as, for example, ethylene glycolor e) water and at least one silane or at least one silanol or at leastone siloxane or any combination of them.

As would be understood to one skilled in the art that also compounds ofother metals or nonmetals or any additives or both of them such as, forexample, PTFE, any organic polymer any e.g. epoxy groups containingpolymer, any lubricant such as, for example, molybdenum sulfide, anysurfactant, any organic solvent like an alcohol, any silane, anysilanol, any siloxane, any polysiloxane, any derivative of thesecompounds or any mixture of these may be incorporated to the electrolytesolution.

The composition of the electrolyte solution may optionally include theat least one solvent besides of water preferably in a totalconcentration in the range from 0.01 to 500 g/L, more preferred in therange from 0.5 to 200 g/L, most preferred in the range from 5 to 50 g/L.

The composition of the electrolyte solution may contain the at least onesolvent besides of water preferably in a total concentration in therange from 0.02 to 25 M/L, more preferred in the range from 0.1 to 10M/L, most preferred in the range from 0.25 to 2.5 M/L.

The electrolyte solution shows a pH preferably greater than 7, greaterthan 8 or greater than 9, more preferred greater than 10 or even greaterthan 11; the pH may especially be in a range from 8 to 14, in a rangefrom 9 to 13 or even in a range from 10 to 12; on the other hand, the pHmay be often below 13 or below 12. The pH is preferably less than 14 orless than 13, more preferred less than 12.

The alkaline pH value is preferably achieved or during the anodizingprocess further adjusted, at least partially, by an addition of at leastone alkali metal silicate or at least one alkali metal pyrophosphate orboth.

The electrolyte solution of the present invention is preferably basic.To increase the pH in some cases, there should not be added or notprimarily added at least one hydroxide component. The method of thepresent invention excludes the use of high contents of alkali metalhydroxides and of ammonium hydroxide in order to ensure that the pH ofthe electrolyte solution is in the desired range without increasing therisk of the early gelling effect of the electrolyte solution. It ispreferred, to add other very alkaline compounds like a pyrophosphate toadjust the pH to higher values. Alternatively or additionally, the pHmay be adjusted by the addition of an amount of “liquid glass”, this iswater glass, which shows a content of at least one hydroxide like sodiumhydroxide or potassium hydroxide or both. It has been found that asignificant addition of water glass does not negatively affect theproperties of the electrolyte solution with such low hydroxide contentsas by addition of a metal hydroxide. Although not wishing to be bound toany theory, it is believed that an increasing presence of at least onealkali metal hydroxide compound, often in ionic form, in the electrolytesolution increases undesirable hydroxide contents e.g. mainly of thehydroxide of the base metal of the metallic surface like mainlymagnesium hydroxide in the coating especially on magnesium rich surfacesand decreases the stability of the electrolyte solution.

The Micro-arc Oxidation Process for Metallic Surfaces:

In the method of treating a metallic workpiece, a pulsed direct current(DC) or an alternating current (AC) preferably be applied as the currentbetween said metallic surface and said electrode. The micro-arcoxidation process of the present invention involves immersing aworkpiece having at least one metallic surface in an electrolytesolution of the present invention and allowing the surface to act as anelectrode of an electrical circuit.

As understood by one skilled in the art, it is necessary to control thepotential of current during the micro-arc oxidation process. If thepotential is very low, no sparking occurs. In contrast, a high potentialleads to excessive heating of the workpiece and provides coatings with alow adhesion. Experiments have shown that effective sparking begins at aminimum of about 60 V. Above about 1000 V, the heating of the metallicworkpiece is intense. As a guideline, a potential from about 70 V toabout 900 V has been found to be preferably suitable for the micro-arcoxidation process according to the method of the present invention.

Also clear to one skilled in the art is that the current density duringmicro-arc oxidation process is changed. The current density on aninitial stage of the process should be high enough to reach a stablemicro-arc regime, e.g. in the range from 15 to 50 A/dm². Then thecurrent density may be decreased by a non-controlled way to about 2-10A/dm² or for example by a controlled decreasing method as e.g. describedin SU 1713990. A stable micro-arc regime means that the plasma layergenerated during the anodizing process is located essentially stable onthe metallic surface going to be coated and is seen without or nearlywithout any change of the plasma light during the anodizing process.

Although the method of the present invention may be performed onstandard anodizing equipment only allowing direct current and in somecases even pulsed direct current, the anode-cathode regime is morepreferable. The ceramic layer obtained in the anode-cathode regime ismore homogeneous and has a higher sintering rate. It is clear to oneskilled in the art that such a sintered ceramic coating according to theinvention has in most cases a higher hardness, a better wear and abetter corrosion resistance than a similar coating generated only withnon-pulsed direct current. In the method of treating a metallicworkpiece, the current applied may preferably be an alternating currentshowing a frequency of the pulses in the range from 1 to 100 Hz, morepreferred in the range from 10 to 85 Hz, most preferred in the rangefrom 25 to 75 Hz, especially in the range from 45 to 65 Hz.

In the method of treating a metallic workpiece, the current applied maypreferably be an alternating current showing a frequency of the pulsesin the range from 10 to 1000 Hz, more preferred in the range from 100 to850 Hz, most preferred in the range from 250 to 750 Hz, especially inthe range from 400 to 650 Hz.

In the method of treating a metallic workpiece, the current density ofthe pulses in the applied pulsed direct current may preferably be variedin the range from 0 to 100%, more preferred starting in the range from 0to 10% and leading up to the range from 90 to 100%.

In the method of treating a metallic workpiece, the voltage of thecurrent applied may preferably be in the range from 60 to 1000 V, morepreferred in the range from 150 to 900 V, most preferred in the rangefrom 220 to 750 V, especially in the range from 300 to 600 V.

In the method of treating a metallic workpiece, there may preferably bean average current density during the application of the current in therange from 2 to 50 A/dm², mentioned only for the process without thefirst ten seconds and without the last about ten seconds of currentapplied for the actual coating process, more preferred in the range from4 to 40 A/dm², most preferred in the range from 7 to 32 A/dm²,especially in the range from 10 to 25 A/dm².

In general, when e.g. aluminum surfaces, magnesium surfaces orcombinations of these are anodized according to the methods known in theart, sparking occurs. The sparking will often form large pores on theanodized surface, e.g. of up to about 0.5 mm diameter, rendering thesurface susceptible to corrosion and for some applications unesthetic.In contrast thereto, when the anodizing of the present invention isperformed in the sparking regime, the pores in the coating generated arevery small, often typically not visible on the surface of the anodizingcoating with the naked eye.

Since the electrical parameters of the anodizing process are dependenton many factors including the exact composition of the bath, the shapeof the bath and the size and shape of the workpiece itself, the exactdetails of the electrical current are not generally critical to thepresent invention and are easily determined, without undueexperimentation, by one skilled in the art performing anodizing asdescribed herein.

According to a feature of the present invention, the current density canbe chosen at any given anodizing potential so as to be sufficient toreach the controlled micro-arc regime—which may occur at a currentdensity especially in the range from 5 to 50 A/dm², often in the rangefrom 8 to 40 A/dm², most preferred in the range from 10 to 30 A/dm².Even the voltage used is often significantly high. To reach a controlledmicro-arc regime, it seems to be primarily necessary to have a specificchemical composition of the electrolyte solution. Therefore, theconditions for a controlled micro-arc regime are quite different fromthose for a controlled micro-sparking regime. During the anodizingaccording to the controlled micro-arc regime, micro-plasma arcs areobserved on the metallic surface to be coated during the anodizingprocess, especially as small sparks, but often all the surface(s) ornearly all the surface(s) to be coated show blue sparks similar to neonlights, typically like a plasma layer e.g. of up to 3 mm height.Typically, the micro-arc regime is dependent on the electrical andchemical conditions, which means for this invention that it isespecially combined with the typical ranges of the current density andof the chemical composition. The term “controlled micro-arc regime” asused herein means that the micro-plasma arcs do not provide burnings inthe anodizing coating which cause damage of the coated workpieces. Thecontrol of the “controlled micro-arc regime” may preferably be carriedout by controlling the current density, the voltage or both togetherwith the control of the chemical composition of the electrolyte solutionlike the pH and the silicon content.

As it is clear to anyone skilled in the art, it is necessary to controlthe potential of the current during the anodizing process. The potentialused for the process according to the invention is preferably in therange from 200 to 1500 V, more preferred in the range from 250 to 1000V, most preferred in the range from 300 to 800 V. A high potential leadsto a strong heating of the workpiece treated. Experiments did show thatan effectively controlled micro-arc regime may often begin at a minimumof about 200 V. Above about 1000 V the heating of the workpiece may insome cases be too intense and may sometimes even damage the workpiece.The smaller the metallic sample that is going to be anodized, thesmaller may be the voltage. As a guideline, a potential in the rangefrom 280 V to 850 V has been found to be mostly suitable for theanodizing according to the process of the present invention. Theseranges are the same for AC and DC applications.

According to a feature of the present invention, the current density maybe chosen so as to be sufficient to reach a controlled micro-arc regime.Generally, this controlled micro-arc regime may be very often reached ata current density in the range from 12 to 25 A/dm² of the surface.

The current regime may preferably be a pulsed anodic direct current oran anode-cathode regime using alternating current. It has been foundthat these two types of regimes are better than a non-pulsed directcurrent because there seems to be a higher content of oxides generatedin the coating, roughly estimated e.g. 80 to 99% of oxides byalternating current, 30 to 70% by pulsed direct current instead of 25 to50% of oxides for non-pulsed direct current-estimated for comparableprocess conditions. Further on, it seems to be favorable to use as faras possible rectangular or essentially rectangular forms of the currentor of the current density or of both for the pulsed anodic directcurrent or for the anode-cathode regime using alternating current. Whenan anode-cathode regime is used, the industrial frequency in the rangefrom 45 to 65 Hz is preferred, especially in the range from 50 to 60 Hz.However, especially a higher frequency may also be well applicable.

The present invention concerns especially a micro-arc oxidation process,especially for surfaces of magnesium rich or aluminum rich surface(s) orfor both types of surfaces or for a mixture of surfaces containingpartially magnesium rich or aluminum rich surface(s) or for both in anelectrolyte solution of the present invention.

Preferably, the temperature of the electrolyte solution is maintainedespecially during said passing of a current, if necessary by cooling orby heating or by both, in the range from 0 to 60° C., more preferred inthe range from 10 to 50° C., most preferred in the range from 15 to 40°C., often in the range from 18 to 35° C.

In the method of treating a metallic workpiece, a coating may preferablybe formed within less than 150 minutes of passing the current throughsaid electrolyte solution, more preferred within less than 80 minutes,most preferred within less than 50 minutes, especially within less than20 minutes.

In the method of treating a metallic workpiece, a coating may preferablybe formed with an average forming rate of at least 1 μm thickness perminute during the time of passing the current through said electrolytesolution, more preferred of at least 2 μm/min, most preferred of atleast 3 μm/min, especially in the range from 4 to 12 μm/min, often ofabout 5 μm/min.

In the method of treating a metallic workpiece, a micro-arc oxidationcoating, a typical anodizing coating or a coating intermediate betweenthese types may preferably be formed. The micro-arc oxidation coatingtypically shows in many cases a higher oxide(s) content thanhydroxide(s) content. The anodizing coating typically shows in manycases a higher hydroxide(s) content than oxide(s) content.

In the method of treating a metallic workpiece, a micro-arc oxidationprocess may preferably be used.

In the method of treating a metallic workpiece, a hydroxide and oxidecontaining coating may preferably be formed.

In the method of treating a metallic workpiece, an oxide rich sinteredcoating may preferably be generated, especially with a content of oxidesin the coating of at least 60% by weight, of at least 70% by weight, ofat least 80% by weight or of at least 90% by weight.

In the method of treating a metallic workpiece, the metallic surfacesmay preferably be selected from surfaces that are at least partiallysurfaces of aluminum, aluminum containing alloys, aluminum alloys,beryllium, beryllium containing alloys, beryllium alloys, magnesium,magnesium containing alloys, magnesium alloys, titanium, titaniumcontaining alloys and titanium alloys, iron, iron containing alloys andiron alloys or any mixtures of them, more preferred they are at leastpartially surfaces of aluminum, aluminum containing alloys, aluminumalloys, magnesium, magnesium containing alloys, magnesium alloys,titanium, titanium containing alloys and titanium alloys or any mixturesof them; most preferred they are at least partially surfaces ofaluminum, aluminum containing alloys, aluminum alloys, magnesium,magnesium containing alloys, magnesium alloys, titanium, titaniumcontaining alloys and titanium alloys or any mixtures of them.

Herein further, the term “magnesium surface” is understood to mean atleast one surface of magnesium metal or of magnesium-containing alloysor of any combination of them. The magnesium alloys include but are notlimited to AM50A, AM60, AS41, AZ31, AZ31B, AZ61, AZ63, AZ80, AZ81, AZ91,AZ91D, AZ92, HK31, HZ32, EZ33, M1, QE22, ZE41, ZH62, ZK40, ZK51, ZK60and ZK61.

Development of the Anodizing Coating

The anodizing coating produced during the anodizing may be produced witha composition of an aqueous electrolyte solution according to theinvention.

While not wishing to be bound to a known theory or mechanism or topropose a new theory or mechanism, it is believed that a formation ofphosphate(s) and silicon containing polymers in the first layer on themetallic surface will mostly occur at the beginning of the anodizing.Then a deposition of said polymers on the metallic surface(s) mayincrease the micro-arc formation and, by this phenomenon, the hardnessof the generated coating may improve. During the anodizing, often firstat least one hydroxide may be forming part of the beginning coatingwhereas this may be partially, mostly or totally transformed to at leastone oxide like at least one silicon oxide, magnesium oxide, aluminumoxide or any mixed oxide or any mixture of them; this coating showing anintermediate stage of the development of the coating is herein called“basic coating”. This basic coating may be improved if there would be asintering later on, preferably if there is a content of at least onecompound containing Al, Ti, Zr or any mixture of these chemicalelements. By sintering this more or less oxide containing coating atelevated temperatures, a ceramic coating will be generated. All thestages during the development of the coating show a continuoustransition and are not clearly separated. It is assumed that a formationof phosphate(s), phosphide and silicon containing oxide(s) and siliconphosphide in the coating will mostly occur. Furthermore, the phosphatecontent in the electrolyte solution may provide a formation of compoundsthat may be water-insoluble or nearly water-insoluble such as phosphatesof aluminum, beryllium, magnesium, iron, titanium or phosphides ofaluminum, beryllium, magnesium, iron, silicon, titanium or any of theirmixtures.

The coating generated during the anodizing process, especially duringthe micro-arc oxidation process, may preferably show a compositioncomprising 1) at least one oxide, 2) at least one phosphorus containingcompound and 3) optionally, but often, at least one hydroxide. Thiscoating may preferably show a composition comprising 1) at least one ofthe compounds selected from the group consisting of silicon oxides,magnesium oxides, aluminum oxides and any mixture of them, 2) at leastone of the compounds selected from the group consisting of phosphates,phosphides and any mixture of these compounds and 3) optionally, butoften, at least one hydroxide.

This coating may preferably be a composition comprising a) at least onephosphate or at least one phosphide or any mixture of these and b) atleast one oxidic silicon containing compound and c) at least onecompound having cations of the base metal of the metallic materialwhereby hereof at least one compound may be identical with at least oneof the compounds of a) or of b) or of both.

Such compound(s) containing at least one chemical element chosen fromAl, Ti, Zr and any mixture of these may penetrate into the coating layerduring the oxidation process, especially compounds in the form ofparticles. The energy of a plasma-chemical reaction on the metallicsurface(s) is necessary for the decomposition of the compounds and forthe oxidation of the metals and enhances then a sintering of themetallic oxides with the basic coating. This method allows to modify thebasic coating and to obtain a variety of coatings with an improvedhardness, an improved thermal resistance and sometimes with improvedother properties like a further reduced porosity, like electricallyinsulation, piezoelectric properties or ballistic shielding propertiesor any combination of them. The content of compound(s) comprising atomsof Al, Ti, Zr or any mixture of these chemical elements is preferably inthe range from 0.1 to 99% by weight of all phases of the coating, morepreferred in the range from 1 to 50% by weight. This indicates, thatsuch atoms may be sometimes distributed broadly in the coating.Additionally, when at least one Zr compound is used, at least onestabilizer like at least one compound selected from the group consistingof alkaline earth metal containing compounds, lanthanide containingcompounds and yttrium compounds may be added to the electrolyte solutionin order to stabilize the generated zirconium oxide. An example of saidstabilizers may preferably be cerium oxide or yttrium oxide. The coatingmay then preferably show a composition comprising at least one compoundcontaining Al, Ti, Zr or any mixture of them.

The generated coating may in many cases be slightly or intensivelysintered as there are often temperatures applied in the range from 1000to 2000° C. during the anodizing and especially during the micro-arcoxidation process. According to first observations, the microhardness ofan unsintered coating on a magnesium alloy may e.g. be roughly about 90to 95 HV₅₀, of a partially sintered coating e.g. roughly about 150 to200 HV₅₀ and of a well sintered coating e.g. roughly about 400 to 450HV₅₀. Even the corrosion resistance seems to be according to firstobservations roughly proportional to the sintering degree: The corrosionresistance by tests in 5% salt fog in accordance with ASTM D117 may e.g.be roughly about few hours for an unsintered coating on a magnesiumalloy, may e.g. be roughly about 240 to 300 hours for a partiallysintered coating on a magnesium alloy and may e.g. be roughly about 1000hours for a well sintered coating on a magnesium alloy. It is estimatedthat the porosity may show a similar development with the sinteringdegree. Such coatings may preferably have a content of at least 70% byweight of at least one oxide compound, more preferred of at least 80% byweight, most preferred of at least 90% by weight. Because of theexcellent results, no sealing is necessary for the well sinteredcoatings.

The coating generating during the anodizing process may preferably gaina coating thickness in the range from 10 to 300 μm, more preferred inthe range from 20 to 250 μm, most preferred in the range from 25 to 190μm, often in the range from 30 to 150 μm, especially at least 40 μm orup to 120 μm, sometimes of about 50 or 60 μm.

It was surprising that excellent coatings showing a very high corrosionresistance even on unsealed surfaces especially of magnesium richmaterials could be gained. All coating generated according to thisinvention that give at least a certain corrosion resistance shall beseen as protective coatings.

It was surprising that for the process according to the inventionelectrolyte solutions could be very successfully used that contain onlyenvironmentally friendly compounds.

It was surprising that excellent coatings could be generated even with acoating rate of at least 3 μm/min, sometimes of at least 6 μm/min,calculated as average over the practically whole anodizing time.

Further on, it was surprising that excellent coatings could be generatedeven within less than 30 minutes, partially even in a time period in therange from 1 to 25 minutes.

It was surprising that a ceramic coating which was well sintered andshowed a typical coating thickness of about 50 μm, an excellentcorrosion resistance and a high microhardness could be gained alreadyafter only 5 minutes of anodizing.

EXAMPLES AND COMPARISON EXAMPLES

The following sections describe specific examples and comparisonexamples with the target to show some of the possible process varieties,composition varieties and the effects related thereto more in detail andnot to limit the invention.

Section 1: Preparation of the Different Electrolyte Solutions and Trialsfor Coating:

In the following, the preparation procedure of the electrolyte solutionsas mentioned in Table 1 is described. An amount of Na₂HPO₄.2H₂O wasdissolved in 500 ml of water. To this solution, an amount of K₄P₂O₇ wasadded and thoroughly mixed. Then, Na₂SiO₃ as water glass was added tothis solution as commercially available “liquid glass” solution andagain thoroughly mixed. Finally, water was added to adjust theelectrolyte solution to 1 liter of an electrolyte solution of thepresent invention. In some of these examples, hydrogen peroxide andsodium aluminate were added. TABLE 1 Compositions and pH values of theaqueous electrolyte solutions of the examples according to the inventionSolution No., Example No. Unit 1 2 3 4 5 Na₂HPO₄ g/L 18 9 7 2 2 K₄P₂O₇g/L 33 16 13 5 5 Na₂SiO₃* ml/L 50 25 20 7 7 H₂O₂ 28% ml/L — — 10 — 5Na₃AlO₃ g/L — — 0.5 — 0.2 Hydroxides added of Na, g/L 0 0 0 0 0 K, Li,NH₄ pH — 11.8 11.5 11.5 11.2 11.3 coating thickness, about μm 47 53 45(50) (50)*as liquid glass = water glass in the form of 20% of this silicate inaqueous liquid with a specific gravity of 1.3 g/cm³, data including thewater content.

First, the plates and sheets of the aluminum respectively magnesiumalloys used for the further process were cleaned in an alkaline cleaningsolution. The coating of these sheets was performed in a cooledlaboratory tank with a stainless steel (SS316) electrode as the cathodeand with direct pulsed current of a voltage of up to 200 V for everysample, with a current density of 10 to 25 A/dm² with the maximumshortly after starting and with a continuous uncontrolled decrease ofthe current density for every sample as well as at a temperature of theelectrolyte solution of about 25° C.

With the compositions according to Table 1, coatings were generated onthe surfaces of the magnesium alloys AZ31B, ZK60 and AZ91D as well as onthose of the aluminum alloys A15053 and A16061 for each solutionmentioned in Table 1 over 5 minutes. All these coatings showed good oreven excellent results depending on the composition of the electrolytesolution. The coatings generated on these magnesium alloys and aluminumalloys showed almost the same coating characteristics one to the otherprepared with these significantly alkaline electrolyte solutions. It wasfurther found that the samples coated in the medium concentratedelectrolyte solution No. 2 according to the invention had a slightlyhigher coating thickness when using exactly identical coating times andshowed a better corrosion resistance than in the examples Nos. 1 and 3.

Comparison example No. 1 in a standard sulfuric acid hard anodizingprocess: Parallel hereto, the aluminum alloys A15053 and A16061 weretested according to the standard sulfuric acid hard anodizingelectrolyte solution in accordance with Mil-A-8625 F Type III Class 1.The coating was generated with a coating thickness of about 50 μm.

Comparison example No. 2 in a standard sulfuric acid hard anodizingprocess: Further on, panels of aluminum A12024 were parallelly theretocoated by a hard anodizing process in accordance with Mil-A-8625 F TypeIII Class 1 and were sealed afterwards in a hot nickel acetate solutionas described in Mil-A-8625 F. These panels showed coatings of about 50μm coating thickness.

Comparison example No. 3 in a conventional alkaline anodizing processfor magnesium rich surfaces showing typically excellent corrosionresistance properties: Finally, panels of the magnesium alloys AZ91D andAZ31B were coated in an anodizing solution number A as described in WO03/002773 for 10 minutes at 25° C. with a current density of between 2and 4 A/dm². This solution was prepared with 0.2 mole of Na₂HPO₄.2H₂Owere dissolved in 500 ml of water. To this solution 25 ml of 50%solution of NH₂OH were added and thoroughly mixed. To this solution wasadded 40 g of KOH and thoroughly mixed. To this solution 0.2 g of thepolymeric surfactant Brij® 97 was added. Water was added to make 1 literof the alkaline anodizing solution. This solution is used and approvedin a more conventional anodizing process with a solution giving coatingsof high corrosion resistance. The coating was generated with a coatingthickness of about 20 μm.

It was found that all the coated panels of the magnesium alloys and ofthe aluminum alloys coated in a solution according to the invention(solutions Nos. 1 to 5) and with a process according to the inventionshowed significantly better results of corrosion resistance and hardnessthan the coatings of the comparison examples Nos. 1 to 3.

Additionally, a coating thickness of 50 microns was obtained with aprocess according to the invention already after 5 minutes of treatmentin the respective electrolyte solution of the present invention. In theanodizing solutions of the comparison examples Nos. 1 and 2, the samethickness was obtained after 40 to 50 minutes of the standard sulfuricacid hard anodizing process.

Section 2: Content of Silicon in the Generated Coatings

The coatings of the panels of the magnesium alloy AZ31B coated asdescribed in section 1 with the solutions Nos. 1 to 3 of Table 1 wereanalyzed on their silicon content. The content of silicon was testedwith an emission spectroscope GDA-750 by Glow Discharge Optical EmissionSpectroscopy. The test was performed in accordance with the QuantitativeDepth Profiling Method (QDP).

Astonishingly, it was found that the samples coated in a mediumconcentrated electrolyte solution (solution No. 2) have the highestsilicon content: 17%. The samples coated in a high concentratedelectrolyte solution (solution No. 1) showed a content of 15% of siliconin the coating. The samples coated in a low concentrated electrolytesolution (solution No. 3) have a content of 12% of silicon in thecoating.

Section 3: Micro-Hardness of the Generated Coatings

Panels of the magnesium alloy AZ31B coated in the electrolyte solutionsNos. 1 to 3 of Table 1 showing a coating thickness of about 50 micronswere tested on their Vickers micro-hardness. All three samples showed ahardness of about 400 HV₅₀. As they showed only about 2 or 3 minor poresto be seen with the naked eye on an area of 0.4 dm²; it is supposed thatthe porosity is only of roughly about 1%. The coatings wereastonishingly dense and solid.

Section 4: Corrosion Resistance of the Generated Coatings

Panels of the magnesium alloy AZ91D coated in the solutions Nos. 1 to 3for 5 minutes at 25° C. with a current density of between 10 and 25A/dm² were used for the corrosion resistance test without any sealingafter the micro-arc coating process. These samples as well as theanodized and sealed aluminum alloy panels of comparison example 2 showedcoatings with a coating thickness of about 50 microns. The sealing ofthe panels of comparison example 2 was an impregnation of the pores ofthe porous anodizing coating. All these samples were tested in 5% saltfog in accordance with ASTM D117 for 1000 hours.

The aluminum alloy sample of comparison example 2 was already heavilycorroded after 300 hours of the test. The magnesium alloy samples showedonly 1 to 3 corrosion pits per panel surface with a diameter of lessthan 1 mm each after 1000 hours to be observed with the naked eye;therefore, they were significantly much more resistant againstcorrosion.

It was very astonishing that the coatings generated with the processaccording to the invention on unsealed magnesium alloys showed a verymuch better bare corrosion resistance than the sealed aluminum alloyalthough aluminum alloy surfaces themselves are much less sensitive forcorrosion than magnesium alloys.

The present invention also relates to vehicles, e.g., aircraft,terristrial vehicles such as cars and trucks, and to electronic devicesincluding the coated products of the present invention. For example, thevehicle will comprise an engine and metal parts as prepared by theinvention. Methods of making products that include the coated productsdescribed herein are also contemplated.

1. A composition of an aqueous electrolyte solution useful for theoxidation of a surface of at least one anodizable metallic material witha pH greater than 6 comprising: i. at least two different phosphoruscontaining compounds showing different anions which are at leastpartially soluble in the aqueous solution used, at least a first beingcalled component a) and at least a second being called component b); ii.at least one silicon containing compound which is at least partiallysoluble in the aqueous solution used; and iii. an amount of at least onetype of cations selected from alkali metal cations and ammonium cations;iv. whereby the electrolyte solution shows a total concentration of atleast one hydroxide of Na, K, Li, NH₄ or any mixture of theseintentionally added to the electrolyte solution below 0.8 g/L or wherebythe electrolyte solution is free of any hydroxide of Na, K, Li, NH₄ orany mixture of these added intentionally.
 2. A composition of an aqueouselectrolyte solution useful for the oxidation of a surface of at leastone anodizable metallic material with a pH greater than 6 comprising: atleast two different phosphorus containing compounds showing differentanions which are at least partially soluble in the aqueous solutionused, at least two of them being called component a) and component b),wherein there is contained a moiety of at least one phosphoruscontaining compound showing oxyanions; an amount of at least onecompound selected from organic silicates, inorganic silicates, siliconcontaining oxides, silanes, silanols, siloxanes and polysiloxanes, theirderivatives or any mixture of them that are sufficiently stable in theelectrolyte solution, essentially non-toxic and water-soluble or atleast partially water-soluble; a moiety of at least one type of cationsof Na, K, Li, NH₄ or any mixture of these; whereby the electrolytesolution shows a total concentration of at least one hydroxide of Na, K,Li, NH₄ or any mixture of these intentionally added to the electrolytesolution below 0.8 g/L or whereby the electrolyte solution is free ofany hydroxide of Na, K, Li, NH₄ or any mixture of these addedintentionally.
 3. The composition of claim 1 wherein the electrolytesolution contains a moiety of at least one primary phosphate, of atleast one secondary phosphate, of at least one orthophosphate, of atleast one condensed phosphate, of at least one pyrophosphate, of atleast one phosphonate, of at least one phosphonite, of at least onephosphite, of at least one derivative of them or of any mixture of them.4. The composition of claim 1 wherein the electrolyte solution contains:as component a) a moiety of at least one primary, secondary or tertiaryphosphate or of at least one derivative of them or of any mixture ofthem and as component b) a moiety of at least one pyrophosphate or of atleast one derivative of it or of any mixture of them.
 5. The compositionof claim 1 wherein at least one of said phosphorus containing compoundsis chosen from the group consisting of K₃PO₄, Na₃PO₄, (NH₄)₃PO₄, K₂HPO₄,Na₂HPO₄, (NH₄)₂HPO₄, KH₂PO₄, NaH₂PO₄, NH₄H₂PO₄, K₄P₂₀₇, Na₄P₂O₇ and(NH₄)₄P₂O₇.
 6. The composition of claim 1 wherein the electrolytesolution contains the at least two phosphorus containing compounds in atotal concentration in the range from 0.2 to 250 g/L.
 7. The compositionof claim 1 wherein the concentration of said component a) in saidelectrolyte solution is in the range from 0.1 to 220 g/L and whereinsaid component b) in said electrolyte solution is in the range from 0.1to 220 g/L.
 8. The composition of claim 1 wherein the electrolytesolution contains a moiety of at least one alkali metal silicate or atleast one of their derivatives or any mixture of them.
 9. Thecomposition of claim 1 wherein the total concentration of the at leastone silicon containing compound in said electrolyte solution is in therange from 0.5 g/L to 70 g/L.
 10. The composition of claim 1 whereinthere is a total concentration of at least one hydroxide of Na, K, Li orNH₄ or of any mixture of them of no more than 0.8 g/L in the electrolytesolution.
 11. The composition of claim 1 wherein the electrolytesolution contains additionally at least one peroxide.
 12. Thecomposition of claim 1 wherein the concentration of the at least oneperoxide additionally contained in the electrolyte solution is in therange from 0.01 g/L to 20 g/L-calculated as 100% of H₂O₂.
 13. Thecomposition of claim 1 wherein the electrolyte solution containsadditionally at least one compound containing atoms of Al, Ti, Zr or anymixture of these atoms or any mixture of these compounds.
 14. Thecomposition of claim 1 wherein at least one water-insoluble compoundcontaining atoms of Al, Ti, Zr or any mixture of these atoms or anymixture of these compounds additionally contained in the electrolytesolution is contained in the form of particles showing a particle sizedistribution for all these particles essentially in the range from 0.01to 20 microns.
 15. The composition of claim 1 wherein the concentrationof the at least one compound containing atoms of Al, Ti, Zr or of anymixture of these atoms or of any mixture of these compounds additionallycontained in the electrolyte solution is in the range from 0.01 g/L to50 g/L.
 16. The composition of claim 1 wherein the electrolyte solutioncontains as solvent water or water and at least one alcohol.
 17. Thecomposition of claim 1 wherein the electrolyte solution contains a totalconcentration of at least one solvent besides of water in the range from0.01 to 500 g/L.
 18. A method of treating a metallic workpiececomprising: providing a metallic surface chosen from metallic surfacesof at least one metallic material that may be anodized; immersing saidsurface in an electrolyte solution whereby the solution may really be asolution, a sol, a gel, a suspension or any mixture of them; providingat least one electrode in said electrolyte solution; and passing acurrent between said surface and said electrode through said electrolytesolution wherein said electrolyte solution is an aqueous solution with apH greater than 6 that has a composition as claimed in claim
 1. 19. Themethod of claim 18 wherein a pulsed direct current or an alternatingcurrent is applied as the current between said metallic surface and saidelectrode.
 20. The method of claim 18 wherein the current applied is analternating current showing a frequency of the pulses in the range from1 to 100 Hz.
 21. The method of claim 18 wherein the current applied isan alternating current showing a frequency of the pulses in the rangefrom 10 to 1000 Hz.
 22. The method of claim 18 wherein the currentdensity of the pulses in the applied pulsed direct current is varied inthe range from 0 to 100%.
 23. The method of claim 18 wherein the voltageof the current applied is in the range from 60 to 1000 V.
 24. The methodof claim 18 wherein there is an average current density during theapplication of the current in the range from 2 to 50 A/dm².
 25. Themethod of claim 18 wherein the electrolyte solution during said passingon a current is maintained at a temperature of between 0 and 60° C. 26.The method of claim 18 whereby a coating is formed within less than 150minutes of passing on the current through the said electrolyte solution.27. The method of claim 18 whereby a coating is formed with an averageforming rate of at least 1 μm thickness per minute during the time ofpassing on the current through the said electrolyte solution.
 28. Themethod of claim 18 whereby a micro-arc oxidation coating, a typicalanodizing coating or a coating intermediate between these types isformed.
 29. The method of claim 18 wherein a micro-arc oxidation processis used.
 30. The method of claim 18 wherein a hydroxide and oxidecontaining coating is formed.
 31. The method of claim 18 wherein anoxide rich sintered coating is generated.
 32. The method of claim 18wherein a coating is generated showing a coating thickness in the rangefrom 10 to 300 μm.
 33. The method of claim 18 whereby the metallicsurfaces are selected from surfaces that are at least partially surfacesof aluminum, aluminum containing alloys, aluminum alloys, beryllium,beryllium containing alloys, beryllium alloys, magnesium, magnesiumcontaining alloys, magnesium alloys, titanium, titanium containingalloys and titanium alloys, iron, iron containing alloys and iron alloysor any mixtures of them.
 34. A protective coating produced by a methodas claimed in claim
 18. 35. The coating as claimed in claim 34 having acomposition comprising
 1. at least one oxide,
 2. at least one phosphoruscontaining compound and optionally at least one hydroxile.
 36. Thecoating as claimed in claim 34 having a composition comprising at leastone of the compounds selected from the group consisting of siliconoxides, magnesium oxides, aluminum oxides and any mixture of them, atleast one of the compounds selected from the group consisting ofphosphates, phosphides and any mixture of these compounds and optionallyat least one hydroxide.
 37. The coating as claimed in claim 34 having acomposition comprising a) at least one phosphate or at least onephosphide or any mixture of these and b) at least one oxidic siliconcontaining compound and c) at least one compound having cations of thebase metal of the metallic material whereby hereof at least one compoundmay be identical with at least one of the compounds of a) or of b) or ofboth.
 38. The coating as claimed in claim 34 having a compositioncomprising at least one compound of Al, Ti, Zr or any mixture of them.39. A coating produced with a composition of an aqueous electrolytesolution as claimed in claim
 1. 40. A method of use of a metallicworkpiece coated with a protective coating produced by the method asclaimed in claim 18 for aircrafts, for terrestrial vehicles or forelectronic devices.
 41. A vehicle comprising a metallic workpieceproduced by the method of claim
 18. 42. An electronic device comprisinga metallic workpiece produced by the method of claim 18.